Fischer−Tropsch Synthesis Using H2/CO/CO2 Syngas Mixtures over a Cobalt Catalyst

The effect of CO2 on Fischer−Tropsch synthesis (FTS) on a cobalt based catalyst had been investigated in a fixed-bed microreactor. Two feed gases, H2:CO:CO2 = 2:1:0 and H2:CO:CO2 = 3:0:1, were mixed in various proportions thus varying the ratio of CO, CO2, and H2 stoichiometrically. The results show that CO and CO2 mixtures can be used as feed to a cobalt catalyst. Comparison of the FTS using different syngas mixtures (CO2/H2, CO2/CO/H2, and CO/H2) shows that: (1) CO2 can be hydrogenated along with CO in the FT reactor over cobalt catalyst, especially in the case of high content of CO2. (2) Hydrogenation of CO2 or CO/CO2 mixture leads to a typical Anderson−Schulz−Flory (ASF) distribution. These could support the hypothesis that CO2 hydrogenation processes might occur with the formation of CO as intermediate. (3) CO feed exhibit the typical two-alpha distribution while CO2 and CO2 rich feeds only exhibit a single-alpha distribution. This may also help us to understand the mechanisms that lead to product distributions in FT with single- and dual-alpha distributions. In spite of the fact that cobalt catalysts are not water−gas shift active, it is shown the rate of hydrocarbon production is maximized at an intermediate composition of the CO/CO2/H2 mixture. The results could have implications for the design of XTL (anything-to-liquids is a process that converts carbon and energy containing feedstock to high quality fuels and products, such as coal-to-liquids, biomass-to-liquids and gas-to-liquids) using cobalt catalysts in that it might be advantageous to keep some carbon dioxide in the syngas feed to the FTS process.